Is the differential pressure before and after the control valve the operating environment or the result of its operation?

Same question; I guess many people don’t even have a basic understanding of these processes

The pressure before the control valve is the pressure of the power medium, while the pressure after the control valve is the pressure required for the process, that is, the operating pressure. I believe the meanings of the first and second points mentioned by the original poster are the same, and both are the result of regulation by the control valve. As for the third point, I don’t understand what it means.

The last edit to this post was made by HEJIYUER on 2015-8-4 at 10:43. The differential pressure value required for calculating the control valve is a process parameter specified by the process engineering team. First of all, it should be understood that differential pressure and flow rate are related to each other; the differential pressure specified by the process team always refers to a certain flow rate. Without knowing the flow rate, it is impossible to calculate the Cv value of the control valve. 1. Sometimes the value they provide is the pressure difference between the upstream and downstream devices; I understand this to refer to things like working conditions, such as pressure and temperature – it’s a value that already exists and has nothing to do with my valve. For example, the fuel gas passes through a control valve to be burned in the burner; the pressure in the main fuel gas pipeline is 5 BarG, and the burner also has pressure specifications such as \"an inlet pressure of at least 3 BarG at a flow rate of 2 T/H.\" So, the Cv value is calculated using the differential pressure of 2 BarG @ 2 T/H as specified in the process. In actual operation, if the pressure in the main pipe decreases, in order to maintain an inlet pressure of 3 BarG at the burner, the control valve must be opened wider. As a result of this, the differential pressure across the valve becomes less than 2 BarG; this ensures that the inlet pressure at the burner remains at 3 BarG, thus maintaining the heat load. 2. But sometimes, some people ask that my valve achieve a specific pressure difference, so that the equipment downstream can operate at an appropriate pressure level. I understand that this pressure drop is supposed to be generated by my valve, as it is a result of the valve’s operation. There’s something wrong with this approach; since it’s necessary to ensure that the devices downstream operate at an appropriate pressure level, why not use a pressure PIC for control? Using the same example as above, when the pressure in the main pipe decreases, you still need to maintain the differential pressure across the valve; wouldn’t that result in a lower pressure at the burner inlet? For the same pipeline, if you use a PIC for pressure control, you must \"let go\" and not control the flow rate ； The same holds true in reverse. In the case of a PIC loop, the control valve operates in response to P to maintain P constant; it does not adjust its opening based on flow rate. 3. Moreover, sometimes they assign a fixed value in such cases, claiming that it is based on energy consumption considerations – they don’t want each valve to experience too large a pressure drop, so they set a similar fixed value, such as 0.05 MPa. This approach helps save energy and also allows the instrumentation specialists to select the appropriate valves. I understand that the process engineers consider this pressure drop as pressure loss, but I asked the valve manufacturers, and they said that they don’t take pressure loss into account when calculating control valves, as the pressure loss is very small. The pressure drop of the control valve = pressure loss. If the process provides flow rate but no pressure difference (or only a very small one), then it is recommended to \"remove the control valve for maximum energy savings\" – just kidding. For example, in low-pressure gas system processes, the pressure drop is ideally kept as low as possible. Yet, there is a need for a means to control the flow rate, and the pressure drop available for the control valve is very small (around 10 KPA). Then choose a valve with the same diameter as the process pipeline. Will the control valve be larger than the pipe? Will there be a problem with the instrument? In my opinion, these three are clearly completely different concepts. Yet, by calculating the condition valve based on the pressure difference before and after in the process conditions (regardless of the specific circumstances), no problems have ever arisen. How is that possible? As long as the Cv value of the control valve is large enough so that the flow rate is sufficient even when it is fully open, the process will not hold you accountable. @jiaguoyun